Virtualization allows multiplexing of an underlying host machine between different virtual machines. The host machine allocates a certain amount of its resources to each of the virtual machines. Each virtual machine is then able to use the allocated resources to execute applications, including operating systems (referred to as guest operating systems). An executable layer that provides the virtualization is commonly referred to as a hypervisor (also known as a virtual machine monitor (VMM)). The hypervisor emulates the underlying hardware of the host computer, making the use of the virtual machine transparent to the guest operating system and the user of the computer. A host machine can accommodate more virtual machines than the size of its physical memory allows. Using virtual memory techniques, the host machine can give each virtual machine the impression that it has a contiguous an address space, while in fact the memory used by the virtual machine may be physically fragmented and even overflow to disk storage.